Background: CBP501, a synthetic duodecapeptide, enhances the cytotoxicity of cisplatin in cell lines and xenografts, including NCI-H226 human mesothelioma. CBP501 increases cisplatin influx into tumor cells through an interaction with calmodulin, and inhibits cell cycle progression by abrogating DNA repair at the G2 checkpoint. Phase I clinical trials combining CBP501 with cisplatin alone or with pemetrexed showed acceptable safety profiles and suggested activity. The most common toxicity of CBP501 is infusion-related urticaria, which is easily managed with diphenhydramine. Methods: Chemotherapy-naive patients with unresectable MPM were stratified by histology (epithelioid vs other) and performance status (PS 0-1 vs 2) and randomized 2:1 to Arm A: pemetrexed/cisplatin plus CBP501 25 mg/m² IV (42 pts planned), or Arm B: pemetrexed/cisplatin alone at standard doses (21 pts planned). Patients continued on treatment until progression or intolerance; no maintenance therapy was delivered. The primary endpoint is progression free survival (PFS) in Arm A; if > 23 of the 42 patients remain free of progression more than 4 months, the combination will be deemed worthy of further study. In addition to standard CT imaging to assess response and PFS, PET scans, pulmonary function tests, and mesothelin levels were performed. Results: Enrollment was completed in October 2011. 65 pts from 14 institutions were randomized, and 63 were treated. Patient characteristics in the two arms were similar and as follows: median age 65, 82% male, 71% epithelioid histology, 8% PS2. Grade 3/4 treatment-related toxicities were uncommon, no different than expected from standard chemotherapy, and comparable in the two arms. 70% of patients treated with CBP501 had infusion reactions, all grade 1-2. The best overall response rate in evaluable patients (modified RECIST, investigator assessed) was 12/37 (32%) (95% CI 18-50) in Arm A, and 3/22 (14%) (95% CI 3-35) in Arm B. The median number of chemotherapy cycles was 5.5 in Arm A, 5 in Arm B. Conclusions: Data on the primary endpoint of PFS in Arm A are maturing and will be presented at the meeting.